attention and cognitive control in aphasia as a whole. However, it is important to reduce the scope of current project, as the locus of specific deficits may vary greatly by linguistic domain. For these reasons, the current project will address these questions at the lexical level.
Relevant research in this domain includes the work Lambon Ralph and colleagues, who have looked at the role of cognitive control in PWA with semantic deficits in a number of studies. One example of this work is Jefferies and Lambon Ralph (2006). In this case-series study, the authors compared 10 patients diagnosed with Semantic Dementia (SD) to 10 PWA with semantic deficits (all >1 year post CVA; 5 of these patients were classified as having transcortical sensory aphasia, while the rest were classified mixed, global, or conduction aphasia). All participants were tested on a series of measures characterizing working memory and executive ability, consisting of forwards and backwards digit span (Wechsler, 1987), the Visual Object and Space Perception battery (Warrington and James, 1991) and the Coloured Progressive Matrices test of non- verbal reasoning (Raven, 1962). PWA participants were also given the Wisconsin Card Sort test (WCS; Grant et al., 1948), the Brixton Spatial Rule Attainment task (Burgess and Shallice, 1996) and the Elevator Counting with and without distraction subtests from the TEA (Robertson et al., 1994) to further characterize attention and executive ability. All participants were also given a series of semantic assessments consisting of the Camel and Cactus Test (CCT; Bozeat et al., 2000), spoken word–picture matching, and spoken picture naming.
similar severity, the pattern of results indicated qualitative differences in the underlying causes. SD patients demonstrated strong correlations between performance on all semantic tasks (CCT, word-picture matching, picture naming), high test-retest consistency for specific items, no significant relationships between semantic and cognitive performance, and performed much more accurately on high-frequency items, which the authors interpreted as evidence for an amodal semantic knowledge deficit. In contrast, PWA semantic performance correlated within task type, but not between task type, which the authors hypothesized was due to the differences in control demands required by different tasks. PWA did not show item frequency effects; instead the
difficulty of establishing a specific semantic relationship or ruling out distracters (as rated in a separate norming study on healthy controls) predicted item accuracy. PWA also demonstrated impaired performance on all attention and executive tasks, and semantic performance was significantly correlated with performance on the Ravens and WCS executive tasks. The authors therefore concluded that semantic processing deficits in their PWA participants were caused by deficits in semantic control that negatively affected their ability to accurately access semantic representations in specific and task-relevant ways.
Lambon Ralph and colleagues have tested this semantic control deficit hypothesis using several other designs as well. Jefferies et al. (2008), found that when semantic aphasia and semantic dementia patients were given progressive phonemic cues, semantic aphasia patients were able to name most targets when cuing reached the point of uniquely identifying them, whereas semantic dementia patients only showed limited benefits of
cuing on high-frequency pictures. The authors claimed that increasing cues raise the activation level for a target compared to distracters, and therefore this pattern of results also supports semantic control deficits in aphasia. Soni et al. (2009) also employed phonemic cuing using the same population, but in this study they contrasted correct word-initial phonemic cues against miscues corresponding to the first letter of category coordinates (e.g., if the target was ‘tiger’, the miscue would be ‘l’ corresponding to ‘lion’). They found that miscues resulted in an increase in error rates compared to correct cues, and a marginal increase compared to neutral cues (beeps). The magnitude of these effects was also significantly correlated with individual performance on executive function and attention measures (WCS, Brixton, TEA elevator counting without distraction). Again, this was interpreted at converging evidence in support of semantic control deficits, which impaired their ability to access specific semantic information in task-relevant ways.
Although these studies reported significant relationships between the extent of semantic control impairment and performance on general ‘nonlinguistic’ measures of attention and executive control, one issue that they did not directly address was the domain specificity of these control processes. In response, Hoffman et al. (2013) looked at the domain-specificity of semantic control in 3 PWA who had already been identified as possessing deficits of this type. They tested these patients and controls on a number of non-semantic linguistic control tasks intended to tax the executive components of shifting and updating (Miyake et al., 2000), including rhyme/phoneme judgment and working memory tasks (N-back, complex span with a dual task component, and alphabetical/
reverse letter list manipulation). They found that two of their patients did significantly worse than controls on all these tasks, but that the third patient only did worse than controls on alphabetizing letter lists. They claimed this task required a greater degree of semantic control than others (such as letters reversed), because it required the use of alphabetical knowledge to direct responses. The authors concluded that at least some aspects of semantic and general cognitive control are dissociable, and went on to tie this with functional localization work that argues for both specialized and "multiple demand" regions involved in semantic control in prefrontal and temporoparietal regions.
Hamilton and Martin (2005) also investigated the domain-specificity of cognitive control in PWA, focusing on inhibitory ability in a single patient with known semantic short-term memory deficits (patient “ML”). They tested this patient on both linguistic (Stroop, Recent Negatives) and nonlinguistic (Antisaccade, Nonverbal Stroop) tests of inhibitory function, which were all assumed to load on a single inhibition factor based on factor-analytic work in typical populations (Miyake et al., 2000). However, they found that ML presented with inhibition deficits in the semantic, but not perceptual domains, indicating at least some domain-specificity of semantic control functions.
So far, this review has focused primarily on the presence and potential role of attention and control deficits in aphasia, without making any great attempts to narrow the scope of discussion to specific aspects or models of attention or cognitive control. The following chapter will introduce a specific model of cognitive control originating in the working memory literature (i.e., Executive Attention; Kane and Engle, 2003), which will be the focus of the current work.
CHAPTER 3